A clean break would require the landlord to rescind the current lease and replace it with a new lease, containing only your boyfriend's name. For that, you will need to speak to the landlord and get him/her on board. The recision must be in writing and signed by all involved.
As an alternative, you may also be able to "assign" your rights and obligations to your boyfriend. An assignemnt would consist of a written agreement between you and your boyfriend and should be drafted by a licensed CA attorney. You should note, however, that an assignement will cause you to remain liable to the landlord for your portion of the rent and any damage to the property for the duration of the lease. This is probably not what you have in mind.
If you are nonetheless inclined to go with an assignment, check the language of the lease. You will likely find a provision dealing with subleasing and assignments.
You should be aware that while restictions placed on the transfer of your leasehold interest may be valid, an absolute prohibition against subleases and assignements violates some rent control enactments, as well as state law.
If your lease provides that assignment is permissible only by consent of the landlord, note that consent may not be unreasonably withheld. Reasonableness is likely to turn on your boyfriend's ability to pay the rent on his own.
If the lease does not mention assignments, you can freely assign your portion of the lease.
Your best bet is clearly recission over assignemnt. I suggest having a licensed CA attorney look at the current lease and evaluate the circumstances, in their entirety, prior to making a decision. You should also have an attorney attorney draft and/or approve all agreements pertaining to either recision or assingment.